Observations

There are two things to note here.
One is that session memory increased as the number of rows updated increased.
Two is that session memory increased in chunks.
Two similar queries that update different numbers of rows therefore may or may not consume the same
amount of session memory.

Gotchas

Just because session memory increases with the number of rows updated we should not assume
updating fewer rows will always lead to reduced session memory usage.
For example, as suggested by the results in the
Dimension References
tutorial, using LIKE in dimension references causes MODEL queries to use the same amount of session memory
regardless of how many rows are actually updated.

Warning: Results on your own systems
with your own data will differ from these results.
Results will even differ from one set of test
runs to the next on the same
machine.
Run your own tests and average the results from multiple
runs before making performance decisions.